The objective is to characterize the protective antigens of the rickettsiae and other selected bacteria which are pathogenic for humans. Structure and functional aspects of the immunomodulation by the cell wall and soluble components of Coxiella burnetii (Cb) are currently being analyzed as potential candidates for subunit vaccines. We have shown that (1) whole cell vaccines of Cb suppress the normal mitogenic response of spleenic lymphocytes to ConA, PHA, and PWM, but a subfraction of Cb did not induce this suppression; (2) at least six surface proteins of Cb are recognized by immune sera from rabbits, guinea pigs, and mice; (3) Cb releases a soluble antigen which forms a circulating immune complex in the sera of infected guinea pigs; and (4) endospore formation is characteristic of Cb during its growth in the phagolysosome of host cells.